Conventionally, there are known inkjet printers having print heads provided with a piezoelectric element (PZT). For such print heads, a pulse voltage corresponding to image information is applied to the piezoelectric element, causing a predetermined distortion of the piezoelectric element. This distortion pressurizes ink inside an adjacent container, or ink channel, and an ink drop is discharged from the ink channel toward a print sheet, thus forming a printed ink dot. A plurality of printed ink dots yields an image on the print sheet.
A printer of this type also creates image gradations by discharging ink drops of different diameters. Different diameter ink drops are achieved by varying the amount of change (degree of distortion) of the piezoelectric elements. Specifically, by reducing the amount of change in the piezoelectric element, an ink drop of a smaller diameter is discharged, and by increasing the amount of change in the piezoelectric element, an ink drop of a larger diameter is discharged.
Interestingly, as the amount of change in the piezoelectric element increases, the speed of discharge of a corresponding ink drop from the print head increases. Therefore, ink drops of different diameters have different travel speeds. Conventionally, piezoelectric elements are driven in constant cycles, regardless of the size of the ink drops to be expelled. As an example of the issues created by this arrangement, where the discharge of ink drops of a large diameter follow the discharge of small diameter ink drops, the distance between the centers of printed dots having small diameters and the printed dots having large diameters is smaller than the distance which is ordinarily obtained. In this way, a shift occurs in the positions of the printed dots due to the difference in the travel speeds of the discharged ink drops. This shift causes a marked deterioration in image quality.
FIGS. 19 through 21 illustrate the deterioration in image quality due to a shift in dot position caused by differing ink drop travel speeds following discharge.
FIGS. 19(a) and 19(b) illustrate the deterioration in image quality due to a shift in dot position in a printer using the dither method. FIG. 19(a) shows a standard dither method, while FIG. 19(b) shows gradations using the dither method when a shift in dot position has occurred. The numbers in FIGS. 19(a) and 19(b) represent a gradation number. Arrow D3 (FIG. 19(a)) represents the direction of scanning of a print head.
Assuming the travel speeds of a large diameter ink drop and a small diameter ink drop are equal, the dot pattern is printed as shown in FIG. 19(a). However, as set forth above, the travel speed of a small diameter ink drop is less than that of a large diameter ink drop. Consequently, as shown in FIG. 19(b), a small diameter ink drop experiences a shift in position in a direction opposite the direction of scanning D3.
Moreover, in actual printing, the different ink drop travel speeds may cause a large diameter ink dot and a small diameter ink dot to overlap in an image having, for example, a gradation level 5, which causes said image to be undesirably lighter than an image having a gradation level 4.
FIG. 20 further illustrates the deterioration of image quality due to dot deformation. Because the travel speed of a small diameter ink drop is less than that of a large diameter ink drop, part of a small diameter printed ink dot may merge with an adjacent large diameter printed ink dot. The result is a gourd-shaped dot. Understandably, if a number of such gourd-shaped dots are printed, the printed image will have poor granularity and appear rough.
FIG. 21 also illustrates the deterioration in image quality when a cyclical pattern is printed. Where lines of small diameter printed ink dots and lines of large diameter printed ink dots are alternately printed, the small diameter ink dots are formed closer to the large diameter dots than desired. This occurrence is obvious given that the travel speed of the small diameter ink drops is less than that of the large diameter ink drops. Consequently, the distances between lines will not be constant and the printed image will have cyclical noise.
The problem and examples described above arise from the differing travel speeds of ink drops having different sizes. At least one technique to equalize different travel speeds using an air flow is well known.
FIG. 22 illustrates both the structure and technique utilizing air flow to equalize ink drop travel so as to establish a constant travel speed. The area that expels an ink drop includes piezoelectric element 201, ink channel 202, nozzle 203, and air flow path 204. Piezoelectric element 201, which is distorted via the application of a pulse voltage, causes the pressurization of ink inside ink channel 202. The ink thus pressurized is discharged through nozzle 203 as ink drop 205. The travel speed of ink drop 205 may be made constant by blowing air into air flow path 204, located in front of nozzle 203, in the manner shown by arrow 206.
While this technique has shown an ability to equalize different ink drop travel speeds, printing devices which incorporate this system are complex, which leads to an increase in the cost of manufacture.